TOPIC: MAIL FROM FRANCE TO CHANNEL ISLANDS, 1784. A QUERY

I have an entire dated 19 January 1784 addressed from Marseille to Guernsey. There are no postal markings/town stamps etc. the only indication of passing through the post are the following entries on the reverse viz '15s[ols], underneath is '4' and underneath that is'19s[ols]'. The writer is J & D Barsault (merchants/traders) and the addressee is Thomas Priaulx (privateer/merchant in 1784 but later founder of the Guernsey Bank).
Questions: (i) What route should the letter have travelled from Marseille to Guernsey? and (ii) What should have been the postage because the tariff list for 1759-91 does not seem to allow a 19 sols ratenunder the section for 'Les Pays Etrangers'.
Can anyone offer advice in resolving this query?
One further piece of information: Thomas Priaulx may well have arranged collection of his mail from France in a clandestine manner by picking it up from the French port from which it would normally have crossed legally to the Channel Islands. the letter concerns the selling of brandy and avoiding excise duty.